Marine transportation vessels, such as container ships and tankers, consume large amounts of fuel. In recent times, the trends of digitalization and performance optimisation have led to the vessels using complex systems that produce large quantities of data related to the vessels' efficiency. However, by far the most critical measurements for evaluating vessel performance are those pertaining to vessel speed and energy consumption. Energy consumption roughly grows as the third power of speed, so it is extremely important to measure the speed accurately. The speed over ground (SOG) may be accurately evaluated using known space-based navigation systems like the Global Positioning System (GPS), but this is not equal to STW in the presence of ocean or tidal currents.
The efficiency of the vessel, and hence for example the attained STW at a certain propeller revolution per minute (RPM), may depend on the hull and propeller condition, which can deteriorate due to biological fouling, paint degradation and other factors, or may improve due to a dry docking or cleaning of the hull. To achieve a nearly real-time hull and propeller performance tracking, an accurate STW is required to distinguish the slowly evolving fouling signal from the noisy background. Also, assessing the onboard efficiency of any navigational operation, the key is to know how much the vessel has gained or lost in speed if the RPM or power is kept constant.
The most common sensor type to measure STW for a vessel is the Doppler Log, which transmits ultrasound pulses from the vessel, and measures the backscatter echo. The frequency shift (Doppler shift) can be utilized to calculate the speed of the vessel through water. However, this technique of measuring STW fails to provide an accurate reading in water with a lesser amount of impurity because the technique calculates the STW based on the backscatter echo from bubbles, biological material, and turbidity in water. Indeed, the regular techniques for measuring STW have issues that are commonly known in the industry, see e.g. BOS, M. (2016), How MetOcean Data Can Improve Accuracy and Reliability of Vessel Performance Estimates, Proceedings of the HullPIC 2016 conference, http://data.hullpic.info/HullPIC2016.pdf.
When examining Doppler Log data, two separate issues stand out. First, due to the aforementioned difficulties, the noise level in the speed measurement is often high, and the speed logs can sometimes behave in a very erratic manner. Secondly, the speed logs experience calibration issues, which means that the long-term average difference between SOG and the measured STW clearly differs from zero. Moreover, the calibration error can change over time, either due to crew recalibrating the STW Log device or due to calibration depending on circumstances, such as sea water temperature. Both issues are visualized in the FIGS. 1 and 2, wherein FIG. 1 illustrates an STW log with erratic behaviour and FIG. 2 an STW log with calibration error.
Alternatively, STW can be approximated using modelling based on propeller RPM alone (as in US 2009/0048726), but this approximation is crude and does not provide an accurate STW. To make the approximation more accurate, additional data relating to wind and sea state is typically used (as in WO 2015/129337). This approach has the downside of depending on many data sources that are more prone to breaking.
Therefore, considering the foregoing discussion, there exists a need to overcome the aforementioned drawbacks of conventional ways to measure the STW for a vessel.